1. Field of the Invention
The present invention relates to a monopole wire-plate antenna. More particularly, said invention relates to monopole wire-plate antennas; and of the type having a ground plane, a first radiating element in the form of a capacity top adapted to be connected to a generator or receiver by a feed wire, and a second radiating element in the form of a radiating conductor wire connecting the capacity, top to the ground plane.
2. Discussion of Related Art
One such an antenna is disclosed in French Patent A-2,668,859.
That antenna is formed of two metal surfaces arranged on opposite sides of a dielectric substrate. One of these surfaces, generally the larger one, constitutes the ground plane and the other surface constitutes the top loading. The antenna is fed via the feed wire, formed of a coaxial probe which passes through the ground plane and the substrate and is connected to the capacity top.
This antenna is characterized by the fact that it has an additional active radiating conductor wire which is parallel with the coaxial feed probe and connects the ground plane to the capacity top. This wire provides a return to ground. Such an antenna is the seat of two resonant phenomena, hence the name double resonant antenna which is at times given to it.
The physical parameters of the antenna, namely the permittivity of the electrical substrate, its thickness, the radius of the feed wire, the radius of the radiating wire, the distance between the two wires as well as the shape and dimensions of the capacity top and of the ground plane may, a priori, be of any value. However, the proper operation of the antenna depends on the relations between these parameters which limit the possibility and impose constraints which are at times difficult to satisfy from a technological point of view.
Thus, in order to obtain good matching of the antenna, there is preferably necessary a substrate of very low dielectric constant (xcex5r less than 2), a distance between the coaxial probe and the radiating wire which is very small as compared with the emission wavelength (d less than xcexo/50) and a radius of the coaxial probe at least five times less than that of the radiating wire. On the other hand, the shape of the capacity top is practically arbitrary and only its surface is of importance. Furthermore, it is preferable from the point of view of the matching of the aerial that its height is relatively great but does not exceed xcexo/18. The shape and the dimensions of the ground plane modify the matching of the antenna only to a slight extent when its surface is at least 10 times greater than that of the capacity top, but they may substantially modify the radiation pattern, as in all monopole radiation antennas.
The operation of this antenna results primarily from a phenomenon of coupling between the feed probe and the radiating wire or no cavity resonance mode takes place.
The addition of the radiating wire under the conditions which will be explained below creates a parallel resonance located at a frequency far less than those of the conventional modes of resonance of a suppressed antenna. A suitable selection of the physical parameters of the antenna makes it possible, on the one hand, to effect a proper matching of the antenna to the conventional generators and receivers, that is to say the antenna has an impedance the real part of which is close to a given value, generally 50 ∩, when the imaginary part is cancelled out, and, on the other hand, to obtain a radiation of the so-called monopole type which has the typical characteristics of the radiation of a monopole, namely:
lobe with symmetry of revolution,
maximum radiation parallel to the ground plane when it is very large and zero in the axis of the wires,
linear polarization with electric field in a plane perpendicular to the antenna.
Therefore, while the antenna described in the aforesaid French patent has the advantages over prior art antennas of being relatively simple in design and construction, of having small dimensions as compared with the wavelength of use, of being capable of being properly;matched with a suitable gain, of having a larger pass band than a conventional suppressed antenna and a stable radiation of monopole type as a function of the frequency, of being able to be used in a network, it has, however, certain drawbacks.
In particular, in order to place the antenna under the conditions of monopole radiation, the dimensions of the wires and the distance between the wires must be much less than the signal wavelength xcex, which is source of technological difficulties and fragility, particularly in microwave. Furthermore, for use in low frequencies, the dimensions, although already far less than the wavelength, are still too large for applications in mobile communication. Furthermore, when the substrate used has a dielectric constant which is too different from 1, the antenna is difficult to match and its pass band is relatively small. Finally, the form of the monopole radiation is not easily adjustable, for instance in order to obtain a greater maximum gain or in order to obtain a larger spatial coverage.
The present invention is intended to overcome these drawbacks.
For this purpose, the object of the invention is a monopole wire-plate antenna comprising a ground plane, a first radiating element in the form of a capacity top capable of being connected to a generator or to a receiver by a feed wire and a second radiating element in the form of a conductor wire conducting the capacity top to the ground plane, the antenna being characterized by the fact that it comprises a plurality of at least one of said radiating elements arranged so that the antenna operates in monopole radiation.
It will be seen from the following that such an arrangement makes it possible to solve the problems which have been pointed out above.
It will furthermore be noted that by the word xe2x80x9cwirexe2x80x9d there is understood not only a conductor of circular cross section but also one of any cross section, such as, for instance, a ribbon. Similarly, the ground xe2x80x9cplanexe2x80x9d, as well as the capacity top or tops, may be formed of curved surfaces, possibly not parallel to each other, in particular in order to generate a monopole radiation of special shape, for instance narrow with a large maximum gain, or wide with a given sector of illumination.
In one particular mode of operation, the characteristics of the antenna, and particularly the shape of the capacity tops are selected in such a manner as to have, at the same frequency or of several close frequencies, an antenna operating both in the monopole mode and in the conventional dipole modes.
Also in one special embodiment, the antenna of the invention has a plurality of conductor wires.
In particular, the antenna of the invention makes it possible to obtain a monopole radiation and a good matching much more easily and with much less stringent technological conditions than in the prior art.
More particularly, the radiating wires may be arranged symmetrically with respect to the feed wire.
In another special embodiment, the antenna of the invention has a plurality of capacity tops, at least one of the capacity tops being arranged to be connected to the generator.
In this latter case, the antenna of the invention can be fed by a coaxial probe which passes through the ground plane, the feed wire of which is connected to a capacity top and the outer conductor of which connects the ground plane to a capacity top located between the ground plane and the capacity top connected to the feed wire.
An antenna in accordance with the invention which comprises several capacity tops may be arranged to present a broad pass band or to present a plurality of resonant frequencies, or to present a monopole radiation pattern close to a given template.
In the special embodiment, the capacity top is substantially rectangular and the radiating wire is connected in the vicinity of the small side of the rectangle.
It has been found that this arrangement makes it possible to decrease the surface and the height with respect to the ground plane. This condition of operation is very important in the case of low-frequency antennas (typically radio antennas), for which the dimensions of the antennas are important.
The feed wires and the radiating wires can also be loaded by circuit elements which are localized or spread out along the wire.
These elements may be passive linear elements (resistor, induction coil, capacitor, any impedance) or active linear elements, but also non-linear elements. Suitably selected, they make it possible, for instance, to decrease the dimensions of the antenna, to change the signal frequency, or to switch several signal frequencies.